It is too broad of a question since character type(tamer, bard, etc) dictates the needs as does playstyle(pvp? luck? etc).
One thing you need to keep in mind is item slot. On a luck suit you are likely to find that people seek out a lucky mempo artifact for one slot which means it's harder to sell any other item for that slot. There are "best in slot" artifacts for several slots on most suit types making it harder to sell non-artifact items for those slots.
Check out the most popular templates for each suit type and pay attention to which slots are typically pre-determined and named artifacts and which slots are random stat drops or playermade. You'll have an easier time selling the non-artifact slot items. Artifacts are popular because they are over cap on various mods.
SO, sorry not to be of more help but change the way you look at suits a bit and your life as a salesman will be less frustrating. I'll start the list of popular non-artifact slots, perhaps others can expand on it.
- Shoulder slot for luck suit is typically 140 or 190 luck playermade, no artifact has more luck for this slot so legendary drop medable luck shoulders sell well.